The present invention relates to an x-ray exposure apparatus usable with a lithography process, for manufacturing highly integrated semiconductor devices more particularly, in a pattern exposure step of such a process.
Because of the demand for the increasing degree of integration of circuits, the width of a line constituting a circuit pattern is now required to be on the order of a micron and further, even sub-micron. To meet this demand, radiation energy having a shorter wavelength is used for printing the circuit pattern, such as ultraviolet radiation, far-ultraviolet radiation and soft X-ray radiation.
As to a lithography apparatus using the soft X-ray radiation, efforts have been made to develop a commercial machine for over ten years, but no success in achieving a practical semiconductor manufacturing device using soft X-ray radiation has been reported as yet. The reasons would be in the difficulties in achieving proper and balanced resolution power, alignment accuracy, throughput, reliability, operability, cost and others.
However, considering in further detail those points, one of the most important problems is with the fact that it is even more difficult to handle the soft X-ray radiation as compared with the ultraviolet radiation. As is well known, the soft X-rays have to be emitted within an evacuated container. Therefore, in order to irradiate with soft X-rays a mask and a wafer which are superposed either in contact with each other or without contact, the soft X-rays are introduced thereto through a window of the vacuum container which is sealed by beryllium foil. The beryllium foil is employed because it absorbs a very small percentage of the soft X-rays. However, it has to be durable against the pressure difference between the high vacuum within the container and the atmospheric pressure under which the mask and the wafer are present, so that approximately 50 microns thickness of the foil is required. The beryllium foil of this thickness absorbs as large as 50% of the soft X-rays, which is disadvantageous.
Another problem stems from divergence of the soft X-rays. When the soft X-rays irradiate the mask and the wafer registered therewith without contact, an image of the mask pattern formed on the wafer may have different dimensions from the original pattern on the mask, if the wafer is inclined with respect to the mask. To avoid this, it is needed to maintain them parallel, but it is extremely difficult to mechanically achieve this. In addition, the divergent rays necessarily result in different projection dimensions between the central area and the marginal area. This requires that the center of the mask be aligned with the axis of the X-ray source for each of the pattern exposure operations, in addition to the alignment between the mask and the wafer.
A further problem is caused by the fact that, when a wafer which has been exposed to a pattern is chemically processed by etching or the like, the wafer is expanded or reduced. This is particularly important when large diameter wafers are used, which is a recent trend or demand, such as 6 inch or 8 inch diameter wafer, since then the expansion or reduction of the wafer can not be neglected. It is, therefore, important to minimize the possible influence of the wafer expansion or reduction in the pattern image.